1. Field of the Invention
The present invention relates to thin walled shaped bodies made of a hydraulically set material, specifically to profiled structural elements. Hydraulically set materials are brittle and feature accordingly a high crushing strength but an extremely low tensile strength and an extremely low bending strength. For this reason thin walled structural elements such as flat and corrugated boards as well as any shaped bodies, which are produced from hydraulically setting materials, such as e.g. cement, are reinforced by an admixing of fibrous materials with the object of increasing their physical properties, specifically their flexural tensile strength.
2. Description of the Prior Art
A fibrous material which has been preferred for decades and has been of excellent service encompassed asbestos fibres. Since, however, the sources of such natural material are continually more limited and since this material gives rise to health hazards, specifically if it is improperly processed or mechanically heavily worn, abraded, efforts have been made worldwide to find or develop replacement fibres for such fibre reinforced cementitious materials, such to replace the asbestos fibres.
All suggestions which hitherto were made did however not provide satisfactory results and, specifically, it has not been possible as yet to find fibres which can properly replace asbestos fibres and indeed secure the requisite long term safety against a rupture of thin walled structural building elements being supported at points having a rather large mutual distance. All hitherto suggested replacement fibres lead to insufficient specific material properties such that it was impossible to meet the minimal safety against fracture prescribed by safety regulations. Accordingly, due to safety considerations even the most promising replacement fibres which are available up to this date can be utilized only by incurring large expenditures regarding the substructure or roofing, such as shortened mutual distances of supporting points, which solution is obviously not acceptable due to economic considerations.
Upon the first appearance of regulations limiting the use of asbestos fibres in civil engineering structures, such as e.g. put forth in Sweden, efforts were made initially to mix asbestos fibres with other natural or synthetic fibres. Mainly due to processing reasons specific mixtures were suggested, which consist of filter fibres and reinforcing fibres as well as an improvement of their bonding properties relative to the hydraulically setting materials by means of chemically pretreating such fibres. Moreover, new fibres were developed which met the demands regarding the reinforcing of cement better than the fibres which were available up to that date. Such new products are e.g. polyacryl-nitrite fibres, such as e.g. "DOLAN 10" (trademark of Hoechst AG, Federal Republic of Germany) as well as PVA-fibres "KURALON" (trademark of the Kuraray Co., Japan) etc. However, none of these fibres were able to secure the strength properties of plates and sheets made from asbestos cement. It is possible that cracks will develop in case of critical applications, e.g. increased local loads on products having large dimensions. Although the new fibres may be successively applied in shaped articles subjected to minor or limited loading forces, they have not been able to lend themselves as yet for an economic application for structural elements such as required e.g. for roof coverings of building structures having large spans, i.e. large distances between individual support areas such as in the range of &gt;0.6 meters.
A common advantage of above mentioned replacement fibres is that they can be processed to hydraulically set building materials in a manner similar to the processing of asbestos in sifting drum machines (e.g. Hatschek-machines), which procedure corresponds to the most widely spread industrial practice. A significant feature of mentioned procedure is an even distribution of the reinforcing fibres within the complete mass. A further feature, however, is the impossibility of arbitrarily increasing the relative amount of the fibres such that regarding the reinforcing effect upper limits prevail.
Further suggestions related to the replacing of asbestos fibres are disclosed e.g. in the European Patent Application No. 0 013 305, according to which fibrillated plastic foils are arranged in a crisscross fashion within the cement matrix. Furthermore, an application of fibrillated film net structures has been suggested (GB-PS No. 1 582 945) as well as the use of embedded steel nets or steel bars as well as any kind of fibres, all such suggestions intended to increase the strength of the building materials. Glass fibres in the form of staple fibres or fibre nets have also been proposed in connection with the suggested fibres. The glass fibres are, however, not sufficiently alkaliproof, such that due to their chemical decomposition it is not possible to secure a long term strength of such glass fibre reinforced structures.
All experiments with reinforcing elements made up to now, which reinforcing elements were not evenly distributed with the cement but rather embedded in the mass, such as nets, bars, etc., made of plastic materials or metal, etc., lead to areas of weakness, which areas favored the forming of cracks, especially supported by the notch effects, and, therefore, led to preprogrammed rupture areas. A significant feature of all hitherto suggested solutions is their low wet strength (tested in accordance with ISO R 393) in comparison with their dry strength (tested in accordance with DIN 274), which is detrimental with regard to their use.